1. Field of the Invention
The present invention relates to an apparatus for detecting the deviation of a reading position of a disc player such as a video disc player or a digital audio disc player.
2. Description of Background Information
Optical disc players are structured such that an optical light beam is used to trace a recording track of a disc which is constituted by depressions called "pits" formed in the recording surface of the disc, and a recording signal is read-out by detecting the light from the recording surface of the disc. For such operations, it is necessary to use various servo systems, namely, a focus servo system for converging the light beam onto the surface of the disc to form a light spot for reading information (information reading point), a tracking servo system for moving the information reading point so that it follows the recording track accurately, and a spindle servo system for controlling a relative speed of the information reading point with respect to the recording disc.
As the focus servo system, systems according to a so-called astigmatic method are known, in which the light beam is converged, by using a cylindrical lens for example, as a focal line in horizontal direction at one of two points on the light path of the light beam which are apart from each other, and at the same time converged as a focal line in vertical direction at the other of the two points, and a light sensor of a quadrant type is disposed at a middle position of the two points, to generate an error signal.
On the other hand, as the tracking servo system, systems according to a so-called three-beam method is known, in which three beams including a main beam for reading information and two sub beams for detecting tracking errors disposed on both sides of the main beam are prepared, and arranged in such a manner that a line passing centers of the three beams forms a predetermined off-set angle with respect to the direction of the track, and an error signal is generated on the basis of a difference between the amount of the two sub-beams reflected by the recording surface of the disc. Since a control range of the tracking servo system is relatively narrow, it is typical to provide a slider servo system for driving a slider carrying the pickup in a direction of disc radius by means of a low-frequency component of the tracking error signal.
Furthermore, the spindle servo system is structured such that the disc is rotated in accordance with a phase difference between a synchronizing signal included in the read-out signal and a reference signal.
As described above, the tracking servo system, the slider servo system, and the spindle servo system are structured to generate an error signal by irradiating a light onto the recording track which is made up of depressions called "pits" formed in the recording surface of the disc, and detecting the light from the recording surface of the disc. Therefore, if the information reading light spot traces a portion where no signal is recorded, erroneous control operations may be pursued so that the servo systems become uncontrollable. In the worst case, mechanisms of the disc player at each part can be destroyed. In view of this problem, conventional disc players have been provided with an apparatus for detecting a deviation of the reading position (a state that the reading position is off a proper position) by detecting the presence or absence of a so-called RF (Radio Frequency) signal, i.e. an information read-out signal, and determining that the reading position of the information reading light spot is off the proper position according to a result of such a detection.
Additionally, it is very often that operations of the above-described various servo systems, and the command of operational modes of the player are controlled by a centralized system using a microcomputer. Especially, in order to determine timings at which the information reading point is operated to jump track-to-track spacings, a pulse signal synchronized in phase with the sync signal of the video signal, and a focus lock signal indicating as to whether or not the focus servo system is in a locking condition are supplied to the microcomputer as control input signals.
However, in the case where the deviation of reading position is determined by the detection of presence or absence of the RF signal, it is necessary to provide devices such as a comparator circuit, and to supply a detection output signal to the microcomputer. Therefore, the centralized control cannot be pursued if the microcomputer does not have an input port for such an output signal. Furthermore, there is another drawback that the scale of circuits becomes large.